Hi,
Could use some help and advice, I need to build an interface between an audiometer and a set of LED lights. The audiometer outputs a dc signal that correlates to the sound reading it registers. To make it nice and easy it outputs 60dB as 0.6v, 70dB as 0.7v and so on.
What I need to build is something that can take that signal and turn on some LEDs in accordance to the output. The LEDs are part of a tower we have already bought and run on 24v and 500mA per set. I need the first set of LEDs to come on at 0.6v the second at 0.7 and the third at 0.8.

Using my rather limited electronics knowledge my first thought was to have a seperate 24V battery for the leds that connect through three seperate transistors and then have the audiometer signal trigger the transistors by having 3 zener diodes block the signal untill the breakdown voltage is achieved.
A few questions on this, is this the best idea? can you get zener diodes that have that low a breakdown?
I have built the rather basic circuit on multisim in an attempt to test the circuit replacing the leds with lights at 12v and while the premise of the circuit works its only at 1.3v 1.4v and 1.5v that the lights come on even though i set the Zeners at 0.6v 0.7v 0.8v. Any ideas why thats the case?
Also how will I calculate what resistors i need?

All help will be much appreciated.

Thanks, Don.

 

Welcome to AAC?

Any ideas why thats the case?

You forgot to allow for the Vbe of the transistors.
If it were me I'd try an LM3914 (or LM3915 if you needed a logarithmic response) in bar mode, with its LED-drive outputs controlling MOSFET-based constant-current sources.

Edit: Btw, doesn't Multisim allow editing of the schematic? The bottom section resembles Hampton Court maze .

 

Welcome to AAC?
You forgot to allow for the Vbe of the transistors.
If it were me I'd try an LM3914 (or LM3915 if you needed a logarithmic response) in bar mode, with its LED-drive outputs controlling MOSFET-based constant-current sources.

Edit: Btw, doesn't Multisim allow editing of the schematic? The bottom section resembles Hampton Court maze .

Thanks for the reply Alec,
I did actually think that the Vbe of the transistors would be the problem but (bare with me here, your dealing with an idiot) would they not all work at the same time? if the voltage wasnt high enough to activate then once all zeners have thiere breakdown voltage topped would all the lights not come on?

Would one LM3914 do the trick or would i need multiple?

Thanks, Don.